Scheme 2. Ligand Screening^,,

The reaction was performed using 1a (0.5 mmol), 2 (1.0 mmol), ligand (20 mol %), Me₂Zn (4.0 mol), Ph₃P=O (20 mol %), and Et₂O (5 mL) under air, and stirred at 0 °C for 12 h.

Isolated yields.

ee values were determined by high-performance liquid chromatography (HPLC) with an OD-H column.